1. Field of the Invention
The present invention relates to a method of gradation display in a picture output apparatus, and particularly relates to a method of displaying gradation in a picture output apparatus in which a picture is divided into picture elements each having a very small area and each of the picture elements is divided into micro picture elements each having a micro area, so that gradation of each picture element is displayed on the basis of the ratio of colored micro picture elements forming a dot to the total number of micro picture elements in the picture element.
2. Description of the Prior Art
Conventional picture output devices such as a duplicator, a printer, a digital copying machine or the like, have implemented a method of artificially displaying gradation in reproducing a picture having gradation. In the method of artificially displaying gradation, a picture is divided into small unit picture elements, so that the gradation is displayed by selectively marking the picture elements dark or light. The method commonly uses regularly arrayed large and small dots as the abovementioned unit picture element.
As such a method using dots, a density pattern method (that is, an area gradation method) has been known. According to this density pattern method, each picture element on a display side (a picture output apparatus side) corresponding to each picture element in an original picture is divided into a plurality of micro picture elements. Some of the micro picture elements, i.e., a predetermined number corresponding to the gradation of the picture element, are selected out of all the micro picture elements in the picture element. The selected micro picture elements are displayed in the colored state with a predetermined color (for example, black). In this method, a dot is formed of the colored micro picture elements in the predetermined number corresponding to the gradation.
In the above-mentioned density pattern method, it is possible to display gradation in steps corresponding to the number of the micro picture elements constituting each picture element on the display side. For example, assuming that the number of the micro picture elements S constituting each picture element is 4.times.4=16 as is shown in FIG. 6, and that each micro picture element S performs binary display, then each picture element can be reproduced with one of the gradation levels the total number of which is (4.times.4)+1=17. That is, let the gradation level when all of the micro picture elements S are white be the zero level, let the gradation when only one of the 16 micro picture elements is colored be the first level, let the gradation level when two of the 16 micro picture elements are colored be the second level, . . . , and the gradation level when all of the 16 micro picture elements are colored be the sixteenth level. It is possible to display the picture element with one of the seventeen gradation levels. Generally, assuming that the number of micro picture elements constituting each picture element is m, the number of gradation levels which can be displayed is (m+1).
As has been described above, each dot is formed of one or a plurality of colored micro picture elements constituting each picture element, and the gradation of the picture element is determined on the basis of the number of the colored micro picture elements forming the dot. The shape of each dot varies depending on the manner in which the micro picture elements to be colored are selected to form the dot, even if the number of the colored micro picture elements is not changed. Consequently, the quality of a displayed picture may vary depending on the shape of each dot.
The manner for deciding the shape of each dot is an important problem, and various methods therefor have been proposed. The methods may be roughly classified into two methods. A first method uses a font type screen generator, in which various shapes of each dot to be formed by one or a plurality of colored micro picture elements of each picture element are properly set in advance corresponding to the respective gradation levels to be displayed. In this first method, since it is possible to independently establish the shape of each dot for every gradation level, it is easy to produce the optimum shape of each dot. However, a memory is necessary for storing the shapes of each dot corresponding to the number of the gradation levels to be displayed. Thus, the memory must store data that is obtained by multiplying the number of micro picture elements constituting each picture element by the number of gradation levels, and it is therefore necessary to provide a memory having a large capacity.
A second method uses a threshold type screen generator, in which the first of a plurality of micro picture elements constituting each picture element is colored for the first gradation level, the first and second micro picture elements are colored for the second gradation level, . . . , and all of the micro picture elements, from the first one to the last one, are colored for the last gradation level. This second method has an advantage in that it requires a small memory capacity because it is possible to produce any one of the patterns of dots corresponding to all the gradation levels if threshold data in the number corresponding to the micro picture elements constituting each picture element are provided. However, since it is impossible to independently establish the shape of each dot for every gradation level, it is difficult to establish the optimum shape of each dot.
In each of the above-mentioned first and second methods, the shape of each dot may vary depending on the manner of selection of the one or more micro picture element(s) to be colored for each gradation level, and the quality of a displayed picture may vary correspondingly.
There have therefore been proposed various kinds of methods of deciding the manner for selection of the one or more micro picture element(s) constituting each picture element be colored for every gradation.
For example, as a method relevant to the above-mentioned second method is disclosed in "Picture Processing Handbook" (edited by the Picture Processing Handbook Editing Commission, published by Shokodo Co., Ltd., June 8, 1987, pages 75 to 76). This reference discloses ways to decide which of the micro picture elements is to be colored, including, for example, to form a spiral shape as shown in FIG. 7(A), to form a Bayer shape as shown in FIG. 7(B), and to form a dot shape as shown in FIG. 7(C). In FIGS. 7(A) to 7(C), each picture element is constituted by a plurality of micro picture elements S.sub.1 to S.sub.16 and the subscripts 1 to 16 of the respective micro picture elements S.sub.1 to S.sub.16 represent the order of coloring the micro picture elements.
The size of the micro picture elements forming dots is generally set near the limit of resolution of a picture output apparatus. If only one micro picture element must be colored to form a dot, or if there is a small projection of a size corresponding to one micro picture element in a group of micro picture elements to be colored to form a dot, it is not easy to reproduce a picture accurately. Consequently, in the case where colored micro picture elements forming a dot are scattered, for example, in the Bayer shape as shown in FIG. 7(B) or the dot shape as shown in FIG. 7(C), there is a problem that the reproducibility of a picture is not stable and the picture quality is apt to deteriorate. If each dot is formed by a mass of colored micro picture elements as in the spiral shape shown in FIG. 7(A), the greater the number of colored micro picture elements, the larger the mass of the colored micro picture elements become, so that the above-mentioned problem is somewhat relieved.
In the above-mentioned spiral method for forming a dot from a mass of colored micro picture elements, however, there is a problem that the reproduction of a picture is unstable for a gradation level of a picture element in which the area ratio of colored micro picture elements to all the micro picture elements constituting the picture element increases to near 50 percent to cause the dot to begin to contact another dot adjacent thereto.
The present invention is based upon the discovery of the inventors that dots adjacent to each other may be kept independent to obtain a relatively stable reproduced picture if there is a distance corresponding to two micro picture elements between the dots adjacent to each other. Moreover, even in the case where dots adjacent to each other are in line-contact with each other, it is possible to obtain a relatively stable reproduced picture. However, if dots adjacent to each other come into point-contact with each other, the reproducibility of a picture becomes suddenly unstable and the picture quality deteriorates.
For example, in the case where dots adjacent to each other are in point-contact with each other as shown in FIG. 8(A), there is a tendency that the dots become completely continuous as shown in FIG. 8(B) or become completely separated from each other as shown in FIG. 8(C). Therefore, the reproducibility of a picture becomes so unstable that the granularity becomes low or the accurate gradation display becomes difficult.
In a method of displaying gradation in which each picture element is constituted by a plurality of micro picture elements having a complicated shape other than a rectangular shape (including a square shape), and provided with a screen angle other than 0 degrees and in which the number of selected (colored) micro picture elements of each picture element is set in accordance with the gradation of the picture element to form a dot, it is impossible to avoid dots adjacent to each other from coming into point-contact with each other at any one of the gradation levels.